Method of producing zinc yellow



iatented Nov. 1 2, 1946 T 7 UNITED STATES PATENT OFFICE METHOD 0F PRODUCING ZINC YELLOW Edwin A. Wilson, Rutherford, and William D.

Newman, Elizabeth, N. J., assignors to Interchemical Corporation, New York, N. Y., a corporation of Ohio No Drawing. Application May 27, 1941',

Serial No. 395,368

Claims. (Cl. 23 5s) I i l 2 (a This invention relates to the pigment known as vide a final pI-I of between about 6.0 and 6.6.

zinc yellow, which is a basic hydrated potassium Moreover, where too little hydroxyl is present, zinc chromate, and has particular reference to a the product is undesirably greenish and dirty in p m al m th d f pr par n t s pigshade, and weak; too much hydroxyl likewise ment- 5 products a dirty weak pigment.

Zinc yellow (generally considered to be In conducting the precipitation, the ratio of K2O-4ZnO-4C1'Os-3H2O) is a pigment which finds hydroxyl to chromate should never be allowed to its principal application in the manufacture of get over 0.875 to 1.0; if this occurs, the ratio of metal paints, in particular priming coats for use zinc to chromate in the final pigment is disturbed, under other paints. The pigment has unusual l0 and poor pigment is obtained. Hence, the prerust inhibiting qualities, apparently due to the cipitation is preferably conducted by adding the fact that it has a very slight solubility in water, three solutions (zinc salt, chrome salt, alkali) so that when water penetrates a paint film, it simultaneously, or by mixing the chrome salt reacts with the pigment to produce a complex and alkali solutions, and then admixing with the which tends to render the iron passive. l5 zinc salt solution. It cannot be prepared by mix- The conventional method for manufacturing ing .zinc and alkali solutions and then adding the pigment is to mix a pigment grade zinc oxide chromate, or by adding a mixture of zinc and with a solution of potassium dichromate and chromate solution to an alkali solution.

then slowly add sulfuric acid. This method pro- The zinc ions can be obtained from any soluble duces a pigment which is useful as a rust-inhibizinc salt-e. g. the sulfate, nitrate, chloride, etc. tive material but, on account of the difliculty in The chromate ion may be obtained by the use controlling the process, the pigment varies conof asolublechromate or dichromate-e. g. potassiderably from batch to batch in color and slum and sodium chromates, potassium and strength. Color and strength are important in sodium bichromates, chromic acid, etc. The some applications of the pigment. Furthermore, hydroxyl ions can be obtained by the use of e p e s pensive. caustic soda or caustic potash, soda ash, am-

Various improvements of this method have monia, etc.; where a dichromate or chromic acid been suggested, such as, for example, the treatis used as the source'of chromate ion, sufficient ment of a zinc hydroxide precipitate in a zinc extra alkali must of course be used to convert the sulfate solution, with potassium dichromate and chromium to chromate, and to yield the necessary potassium chloride; but these proposed processes free hydroxyl for the reaction.

seem to have no substantial advantage over the Typical examples of our invention-are the folconventional method. lowing;

' We have discovered that a pigment zinc yellow Example 1 of unusually good pigment properties can be obtained simply and economically by a direct pre- A solution of 640 pounds of potassium bicipitation of a solution of a soluble zinc salt with chromate and 300 pounds of caustic soda in 600 asolution of a soluble chromate, in the presence a D O Water at F: is placed in a Striking of potassium and hydroxyl ions. In order to get tank prov W an agitation The D 0f the a material with satisfactory pigment properties, 40 solution is 12.1. To this solution is added a solu- 1 the ratios of the reacting ingredients are kept tion of 804 p s of ydrous Z c Sulfate in within rather narrow limits. The same method about 400 ga lons Of Water 0 t gallons) can be used for an essentially similar pigment in at about 75 F. When the zinc sulfate is all in,

which the potassium is replaced by ammonium in the pH is about 6.2; the color of the pigment is the pigment. reddish yellow, and a substantial quantity of Using zinc as a basis, we have found that in chromate ion is free in solution. The mixture is order to get satisfactory pigment properties, there stirred for about 15 hours; t e excess of chromate must be reacted from 0.8 to 1.0 chromate ions 8. 051? disappears, and the precipitate becomes for each zinc ion, and at least 0.5 potassium ion y ll w and lighter- ThiS p on filtration, for each zinc ion (or of ammonium ions to make Washin an drying. W be Commercially the ammonium pigment). Furthermore, from form from batch to batch, having reasonably 0.5 to 0.875 hydroxyl ion should be used for each unifor tinting strength and color, and approxichromate ion. If these limits of hydroxyl to mating the Composition 20' nO" CrOs' I-I2O.

chromate are exceeded in either direction, poor The ingredients in the above example yield yields are obtained due to the solubility of the about 5 molar equivalents of zinc ions, 4.35 molar product in substantially acid solutions (pH below equivalents of potassium ions, 4.35 molar equivabout 6.0), and in alkaline solutions (pl-I above alents of chromate ions and 3.15 molar equivaabout 7.0).; our preferred ratios of reactants prolents of hydroxyl ions (2.1 mols of dichromate 3 reacting with 7.5 mols of caustic soda to give 4.35 of chromate and 3.15 of residual caustic soda); the ions ratios are about as follows 4.35 Chromate to 21110 -=0.87 to 1.0

. 4.35 Potassium to zinc -08? to 1.0

3.15 Hydroxyl to chromate -0.725 to 1.0

Example 2 Pounds KzCrzOv 690 NaOH 300 are dissolved together in 600 gallons of water at 85 F. (pH 12.2).

Strike in 30 minutes with 807 pounds of zinc sulfate in 500 gallons of water at 85 F. (pH 3.0).

Stir until equilibrium is reached (8-15 hours).

Press and wash in press. Dry at 200 F. pH at end of strike 6.3. pH at end of stir 6.6.

Obviously since more NaOH is used than is required to convert all of the K2Cr2O1 to normal chromate, the same solution could he arrived The alkalinity may also he arrived at by a partial or complete substitution of the hydroxide by carbonates.

All of the above mixtures give substantially the same molar equivalents of zinc ions (5.0) potassium ions (4.70) chromate ions (4.70) and hydroxyl ions (2.8). The ratios are Chromate to zinc 0.94 to 1.0 Potassium to zinc 0.94 to 1.0 Hydroxyl to chromate 0.56 to 1.0

Example 3 Pounds NazCrzQv-2Hz0 700 KOH 264 NaOH 112 are dissolved together in 600 gallons of water at 85 F. (pH 12.2).

Strike in 30 minutes with 681 pounds of zinc chloride in 500 gallons of water at 85 F. (pH 3.0).

Stir until equilibrium is established.

Press and wash in press. Dry at 200 F. pH at end of strike 6.3. pH at end of stir 6.6.

The ratios of ions are substantially equal to these of example 2. Changes can obviously be made in the specific examples without departing from the spirit of the invention, which is defined in the claims.

We claim:

l. The method of making zinc yellow which comprises preparing a solution of a soluble zinc salt, preparing a second solution containing chromate ions, and a third solution containing hydroxyl ions, one of the solutions containing potassium ions, in the ratio of at least 0.5 molar equivalent of potassium and from 0.8 to 1.0 molar equivalent of chromate per molar equivalent of zinc, and 0.5 to 0.875 molar equivalent of hydroxyl per molar equivalent of chromate, and

passing the solutions simultaneously and in the above molar ratio of ingredients into a precipitating tank, whereby zinc yellow of high color value and good color is precipitated.

2. The method of making zinc yellow which comprises directly precipitating a solution of a zinc salt with a solution of soluble chromate in the presence of hydroxyl ions and of alkali ions including alkali ions selected from the group consisting of potassium and ammonium ions, characterized by the use in the reaction of 0.8 to 1.0 molar equivalent of chromate ions, and at least 0.5 molar equivalent of alkali ions of the group consisting of potassium and ammonium ions, per molar equivalent of zinc ions, and from 0.5 to 0.875 molar equivalent of hydroxyl ions per molar equivalent of chromate ions, precipitation of zinc hydroxide and highly basic zinc chromates being avoided by obtaining first precipitation in the reaction in a mixture where the molar ratio of hydroxyl ions to chromate ions is between 0.5 and 0.875 to 1.0, and by maintaining the molar ratio of hydroxyl ions to chromate ions below 0.875 to 1.0 throughout the reaction.

3. The method of making zinc yellow which comprises directly precipitating a solution of a zinc salt with a. solution of a soluble chromate in the presence of hydroxyl ions and of alkali ions including potassium ions, characterized by the use in the reaction of 0.8 to 1.0 molar equivalent ofchromate ions, and at least 0.5 molar equivalent of potassium ions, per molar equivalent of zinc ions, and from 0.5 to 0.875 molar equivalent of hydroxyl ions per molar equivalent of chromate ions, precipitation of zinc hydroxide and highly basic zinc chromates being avoided by obtaining first precipitation in the reaction in a mixture where the molar ratio of hydroxyl .ions to chromate ions is between 0.5 and 0.875 to 1.0, and by maintaining the molar ratio of hydroxyl ions to chromate ions below 0.875 to 1.0 throughout the reaction.

4. The method of making zinc yellow which comprises preparing a solution of a soluble zinc salt, preparing a second solution containing .chromate ions, alkali ions including potassium ions, and hydroxyl ions, in the ratio of at least 0.5'molar equivalent of potassium and from 0.8 to 1.0 molar equivalent of chromate per molar equivalent of zinc, and 0.5 to 0.875 molar equivalent of hydroxyl per molar equivalent of chromate, and adding the zinc solution to the alkaline chromate solution, whereby zinc yellow of high color value and good color is precipitated. 5. The method of making zinc yellow which comprises preparing a solution of a soluble zinc salt, preparing a second solution containing chromate. ions, alkali ions including potassium ions, and hydroxyl ions, in the ratio of at least 0.5 molar equivalent of potassium and. from 0.8 to 1.0 molar equivalent of chromate per molar equivalent oizinc, and 0.5 to 0.875 molar equivalent of hydroxyl per molar equivalent of chromate, and passing the solutions simultaneously and in the above molar ratio of ingredients into a precipitating tank, whereby zinc yellow of high color valueand good color is precipitated.

' EDWIN A. WILSON.

WILLIAM D. NEWMAN. 

